1. Field of the Invention
The present invention relates to the use of a copolymer, in which at least one of the monomers has a surface-active structure, as a dispersing agent or aid for crushing mineral substances in aqueous suspension, regardless of the nature of the mineral substance.
2. Discussion of the Background
It has long been common practice as a means of preparing industrial products for use in the paper industry, particularly as a filler or for coating the paper, to make up aqueous suspensions of different mineral substances or mineral pigment fillers, whose particles have surfaces with different affinities to water.
The first category of mineral substances includes mineral substances with a charged hydrophilic surface such as the natural or synthetic calcium carbonates, for example, particularly chalks, calcites or marbles, the dolomites or kaolins as well as calcium sulphate or titanium oxides, satin white, aluminium hydroxides and others. The second category includes mineral fillers with a hydrophobic surface such as talc, mica and others, for example.
These two types of mineral substances do not exhibit the same theological behavior when placed in suspension in water, particularly when prepared in high concentrations for supply to paper factories. However, they must exhibit the same quality criteria for the user, i.e. the Brookfield viscosity thereof must be such that there will be no risk of sedimentation or hardening of the settled mineral substance particles in order to ensure that they will be easy for the user to handle even after storage in tanks for several days without agitation. Furthermore, these suspensions must have as high a content of mineral substance as possible in order to reduce all the costs inherent in transportation due to the quantity of water present.
Prior to the present invention, the dispersing agents or crushing aids conventionally used for mineral fillers with a hydrophilic surface are polyacrylates with a low molecular weight or with a specific viscosity of less than or equal to 25 (EP 100 947, EP 542 643, EP 542 644). However, these agents have a disadvantage in that they are not very efficient when placing in suspension and/or crushing hydrophobic substances, such as talc or mica, which are commonly used alone or in mixtures.
The person skilled in the art is familiar with other agents which can be used for this purpose, these being copolymers in which one of the monomers has a surface-active structure (EP 0 003 235, EP 0 215 565). These copolymers, however, have a disadvantage in that they are not efficient when it comes to placing in suspension and/or crushing mineral substances with a hydrophilic surface such as the calcium carbonates or kaolins, calcium sulphate or titanium oxides.
Consequently, until now, in order to disperse and/or crush a mineral substance with a hydrophobic surface, it has been extremely difficult for the skilled person to use a dispersing and/or crushing agent known for its efficiency in dispersing and/or as an aid for crushing mineral substances with a hydrophilic surface and vice-versa.
Faced with this problem of using dispersing agents and/or a crushing aid specific to a type of mineral substance and after a good deal of research, the applicant has found that selecting the radical of the hydrophobic chain of the surface-active monomer allows the copolymer to be used as a dispersing agent and/or crushing aid irrespective of the nature of the surfaces of the mineral substances to be dispersed and/or crushed.